The present invention relates to real time control of the clearance angle of a veneer lathe knife during the veneer peeling operation. More particularly, the knife clearance angle is continuously controlled as the diameter of the rotating block decreases, in a manner which reduces disruption of the veneer peeling operation.
In a conventional veneer lathe, the knife holder beam which supports the knife and the knife backing element (called the "nose bar") are mounted on a carrier such that the beam may rotate, relative to the carrier, about an axis which is aligned essentially coincident with the knife's cutting edge. Such rotatability is achieved by using bearings to hingedly mount the beam onto the carrier. The outer bearing races are mounted to the end posts of the carrier, with the beam being supported at its ends by the inner bearing races, so aligned that the tip of the knife is situated at the center of the bearings.
To control the angle of the beam relative to the carrier, a hydraulic or pneumatic actuator is mounted between these knife support elements so that they may be controllably rotated relative to one another. The specific actuator used may be selected from a group of different eccentric, screw or hydraulic actuators. For example, one may employ a hydraulic actuator having a single-stroke cylinder whose piston is arranged to urge the massive knife holder beam against gravitation to the start position of the veneer peeling operation and from which position the beam is then allowed to rotate backward by gravitation in a controlled manner by reducing the pressure in the hydraulic cylinder. Such an apparatus is described in, e.g., U.S. Pat. No. 5,143,129.
Control of the above-described hydraulic actuator cylinder is conventionally implemented by a microprocessor-based controller which continuously compares the actual tilt angle of the knife holder beam with the angle the beam should ideally have as the beam and knife are incrementally advanced during the peeling operation. The controller's memory stores a plurality of different, experimentally determined control curves, one for each wood species to be handled by the veneer lathe in question. Each curve comprises a sequence of set points, with each set point defining the ideal knife holder beam tilt angle for the particular wood species at a particular diameter of the block. If the actual value of the knife tilt angle lags behind the set point which defines the ideal angle for the particular wood species and block diameter, then a control signal is applied to the hydraulic actuator cylinder to controllably reduce the cylinder pressure until the actual value of the knife tilt angle becomes, within the tolerance of the control system, equal to the desired set point.
The above-described prior art control system operates satisfactorily, provided that parameters such as wood density, moisture and temperature of the block do not differ significantly from the corresponding parameters prevailing during generation of the experimentally determined control curves. Significant deviation of such parameters can affect the ideal knife tilt angle and disrupt the veneer peeling operation, necessitating prompt manual corrective intervention by the lathe operator, who typically reacts, based upon experience, by changing the knife clearance angle to make it more negative.
If the knife clearance angle is allowed to become excessively positive then vibrations may occur in the knife. These vibrations, which are transmitted to other parts of the lathe, have been found to have a frequency in the range of about 50-200 Hz. The onset of such vibration in the lathe is a clear indication that the knife clearance angle is excessively positive and should be reduced. The present invention detects the onset of such vibration and initiates appropriate control action to minimize disruption of the veneer peeling operation.